feat: skip ninja-style ModelSchema classes instead of drawing them empty

[FabianClemenz]

Summary

Part of #171 — the cheap, honest part. #171 stays open; this does not resolve it.

The bug this fixes

Naming ModelSchema via --base-classes produced this:

entity "user" as User {
  primary_key(id) : int
  column(name) : str
  column(email) : str
}

entity "user_schema" as UserSchema {
  ' (no fields)
}

An empty box beside the real table. django-ninja's ModelSchema (inner Meta) and ninja-schema's (inner Config) take their fields from a Django model, leaving the class body empty, and erdify does not resolve that reference. An empty entity is worse than no support — the diagram stops being incomplete and starts being wrong, with nothing telling the reader why.

Now

Warning: skipped UserSchema - its fields come from Meta.model (User), which erdify
  does not resolve. Drawing it would add an entity with no fields.
  See https://github.com/devsuit-berlin/erdify/issues/171

The Django model still renders; the misleading box is gone; the reader is told what happened and where to say something about it.

Detection is deliberately narrow

Only an inner Meta/Config that assigns model counts. Two things that must not trigger it, both tested:

• an ordinary Pydantic model with a nested class Config: frozen = True
• Django's own class Meta: db_table = "user" — which also never reaches this branch, since it classifies as django, not pydantic

A schema that declares some explicit fields alongside Meta.model is also skipped: the entity would still be missing the derived ones, so a partial box is the same problem in smaller form.

Why not implement the rest of #171

I'd hold, and the reasons are worth recording on the issue:

• The design question is unanswered and it is the hard part. A UserSchema is a projection of User, not a table. Drawn as a second entity it duplicates fields and implies a relationship that does not exist. The only defensible shape is an opt-in view, which is a lot of machinery for something nobody has asked to see.
• It is substantially more work than the --base-classes half. Resolving Meta.model to a Django model that is also in the scan, reusing the Django field extraction, applying fields/include/exclude/optional, two incompatible conventions (Meta + fields vs Config + include), and depth, which recurses into related schemas.
• There is no demand yet. Support django-ninja schemas: recognise Pydantic bases defined outside the scanned files #171 has zero reactions and zero comments, and 0.14.0 shipped the first half hours ago.
• It may be the wrong artifact. "What the API exposes vs what the database stores" is a genuinely useful diagram — but that is an API-contract diagram, not an ERD.

This warning is the thing that makes the unknown resolvable: people who hit it now have a reason to comment on #171, and what they ask for is exactly what is missing.

Type of change

• Bug fix
• New feature

Verification

7 new tests (348 total, was 341), coverage 93.12% against the 90% gate: both inner-class forms, a qualified model = accounts.User reference, a partial schema, an ordinary Pydantic model with a nested config, a Django class Meta, and the CLI path.

uv run pytest                 348 passed
uv run ruff check / format    clean
uv run mypy src/              Success: no issues found in 9 source files
mkdocs build --strict         passes
pre-commit run --all-files    clean

Docs: the Bases defined outside the scan note in usage/cli.md now describes the real behaviour and puts the open design question to readers; frameworks/limitations.md gains a row.

One judgement call to check

The warning prints unconditionally, including for library callers. The hint_unmatched_model_packages flag exists precisely so a discovery hint stays quiet for programmatic use — but this is not a hint, it is "I found something and deliberately did not draw it", which is closer to the unconditional parse-error warnings already in parse_all_models. Happy to gate it behind the same flag if you would rather the library stayed silent.

Checklist

• Tests added or updated for the change
• uv run pytest passes locally
• uv run ruff check src/ tests/ and uv run mypy src/ pass
• Docs/README updated if behavior changed
• CHANGELOG.md updated under [Unreleased]